In printing processes such as flexography, presses are used to transfer printed images to a substrate such as paper or plastic film. To accomplish this, printing plates are mounted to cylinders of specific diameters to achieve the desired length or xe2x80x9crepeatxe2x80x9d of the printed image. Thin sleeves have been used over the years as plate carriers to keep jobs mounted for repeated use. The sleeves are mounted onto cylinders, typically by expanding the thin sleeve via air pressure supplied to the cylinder interior. Upon removal of the pressurized air, the thin sleeve contracts and grips the cylinder, thus forming an integral unit. In recent years, repeat builders or xe2x80x9cbridge mandrelsxe2x80x9d have been used to reduce the number of costly cylinders yet still achieve the repeat diameters required. These bridge mandrels tend to have a means of expanding over the base cylinder via air and gripping the base cylinder, after air removal. Another approach is to hold the bridge mandrel in place with hydraulic pressure. These bridge mandrels have various materials in place to provide the desired thickness. A means for supplying air to the outer diameter of the bridge mandrel is also provided to enable expansion of the thin carrier sleeve. The air supply typically passes through various layers of the laminated bridge mandrel structure. Further, a new type of press has been developed wherein the cylinder is fixed to the press in a cantilevered fashion with a removable bearing support on the opposite end to facilitate loading and unloading of bridge mandrels as well as thick sleeves. This type of arrangement is often referred to as a xe2x80x9cfixed mandrelxe2x80x9d press.
Bridge mandrels employing the prior art have been made from various materials. As weight became more critical, lightweight composites became the preferred material.
The approach generally taken in forming bridge mandrels is to produce an inner sleeve laminate made from a low viscosity, reinforced, thermoset resin material. Once the inner sleeve is fully cured, the sleeve may be machined to produce a smooth outer surface. A compressible foam material may then be bonded around the sleeve to facilitate expansion of the base sleeve. To prevent resin from penetrating the foam layer, various barrier materials are applied to the foam layer. A solid, rigid foam material is then applied to the surface of the sleeve/foam assembly, typically using an injection process. Once the foam layer has been applied, the surface can be machined to enable application of the final outer laminate that is typically comprised of a low viscosity thermoset material, either with or without reinforcement, which serves as the carrier for the outer thin sleeve. Air is supplied to the bridge mandrel by mounting inserts to the ends of the unit followed by drilling intersecting holes from the outer surface. Another method involves drilling holes through the laminate to allow air to pass from the inside to the outside of the bridge mandrel when supplied to the fixed mandrel.
There are several deficiencies with the prior art. First, the use of low viscosity, thermoset resins requires the use of multiple manufacturing steps to prevent the resin from penetrating into undesirable areas such as the compressible foam layer. Multiple steps are also required because the low viscosity resin systems tend to move and deform when the layers are applied all at once, causing buckling or waviness in the laminates. Second, the ends of the bridge mandrel are typically fully exposed, thus providing the opportunity for inks and solvents to be absorbed into the various layers of the laminate. This can lead to swelling of the bridge mandrel and a change in the diameter that leads to poor print registration and inferior print quality. Third, the ends of the bridge mandrel are easily damaged because the composite materials used tend to have low impact strength. A related problem is that a notch or key-way is often required on the inner diameter of the inner sleeve to position the bridge mandrel for print registration. Attempts have been made to employ metal inserts for this notch because composites are easily damaged through repeated impact with the pin on the fixed mandrel that must align with the notch. However, this approach tends to have a limited life since the composite is not well suited towards holding the metal insert securely for the life of the bridge mandrel. Finally challenges exist in preventing the air from going into the laminate layers and causing delaminations due to the porous nature of the materials used.
An objective of this invention is to provide a bridge mandrel that overcomes the above deficiencies.
In accordance with this invention the bridge mandrel body comprises a multi-ply inner laminate wrapped around a forming mandrel. An intermediate layer comprised of a rigid material is wrapped around the inner laminate to build thickness. An outer multi-ply laminate is wrapped around the intermediate layer to form the outer sleeve carrier laminate.
In accordance with one aspect of this invention the various components are mounted together on the same support or forming tool. The assembly is then inserted as a unit into an oven allowing it to be co-cured. This enables each laminate to experience the same thermal history and thus minimize conflicting thermal stresses.
In accordance with another aspect of this invention the inner laminate includes layers made from high viscosity thermoplastic material to control resin viscosity thus controlling resin flow.
In accordance with another aspect of this invention the rigid intermediate layer is made from segmented foam having generally radial gaps between adjacent pairs of side-by-side segments. Preferably, the radial surfaces of the segments are coated with a thermoset adhesive.
In accordance with another aspect of this invention the co-cured multi-ply bridge mandrel body is machined at the ends to accept end caps or xe2x80x9cheaders xe2x80x9d that are bonded to the unit. These headers could include air passageways for effecting the expansion of an outer thin sleeve that would serve as the printing plate carrier to be mounted to the bridge mandrel for printing. The headers also serve to close off and protect the ends of the bridge mandrel laminate assembly from ink or solvent penetration and from damage due to mishandling during use.